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An environmental study by Caltrans that is supposed to clear the way for widening a 1-mile road through a North Coast redwood grove contains flaws - such as failing to notice a large tree 5 yards from the roadside - a federal judge says.

U.S. District Judge William Alsup of San Francisco, who had previously blocked the roadwork at Richardson Grove State Park in Humboldt County, ordered Caltrans on Wednesday to correct errors in its survey and reconsider its conclusion that construction would cause no environmental damage.

In his injunction in July, Alsup said the project, which had been scheduled to start in January, could endanger redwoods that soar 300 feet above the road and are thousands of years old.

He refused to lift the ban Wednesday.

U.S. 101 narrows to a winding, two-lane road, bordered closely by redwoods as it enters the park 200 miles north of San Francisco. With a few exceptions, long freight-hauling trucks are barred from the road because of safety concerns, requiring lengthy detours - 446 miles extra on a round trip from Oakland to Eureka, Alsup said.

Caltrans said its plan would require cutting down 54 trees, including six redwoods - none of them old-growth - and would remove and replace soil from the root areas of an additional 68 large redwoods. Caltrans concluded that the construction would cause no significant environmental harm.

But Alsup said U.S. Magistrate Nandor Vadas, in a recent visit accompanied by representatives of Caltrans and environmental groups, looked at a sampling of six trees and found discrepancies: The Caltrans study had understated the diameter of one redwood by 20 inches and left out another redwood that stands less than 15 feet from the road.

Caltrans has also admitted, in response to public comments, that it may have underestimated the likely damage to tree roots, Alsup said. The errors "raise serious questions about whether Caltrans truly took a hard look at the effects of the project and made an informed decision," he said.

The judge told the agency to correct its findings and reconsider its conclusions, but stopped short of ordering a broad new environmental review. Opponents said it was time to scrap the project.

"Caltrans wants to cut through, injure and pave over the roots of giant redwoods in a state park for the sake of a few more oversized trucks speeding through the grove, and expects us to believe there won't be any damage," said Peter Galvin of the Center for Biological Diversity.

Caltrans spokesman Scott Burger said the agency will comply with Alsup's orders.